PM / devfreq: exynos-ppmu: Update documentation to support PPMUv2

This patch updates the documentation to include the information of PPMUv2.
The PPMUv2 is used for Exynos5433 and Exynos7420 to monitor the performance
of each IP in Exynos SoC.

Cc: MyungJoo Ham <myungjoo.ham@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>